Generally, a fuel cell is an apparatus for converting chemical reaction energy between hydrogen extracted from a fuel and oxygen in the air into an electrical energy and generating a clean power. The fuel cell is different from a battery in a point that it is possible to continuously generate power as long as fuel is supplied without recharging the power.
Therefore, the fuel cell has drawn much attention as a power source for next generation engines capable of replacing the current internal combustion engine in all industrial fields with an environment friendly power source.
Among the fuel cells, the PEMFC (Proton Exchange Membrane Fuel Cell) is a fuel cell using a high molecular membrane having a hydrogen ion exchange characteristic as an electrolyte, and has been also called an SPEFC (Solid Polymer Electrolyte Fuel Cell) or PEMFC (Proton Exchange Membrane Fuel Cell). The PEMFC has a lower operation temperature of 80° C. as compared to other types of fuel cells, and has a high efficiency and a large current density and output density, and has a short start time, and a fast response characteristic with respect to a load variation.
In addition, due to the use of high molecular membrane as an electrolyte, it is not necessary to adjust corrosion or electrolyte for the PEMFC and it is also shown not sensitive to variation in pressure of a reaction gas.
In addition, the simplicity in design and easiness in fabrication make it possible to implement various kinds of outputs. Therefore, the PEMFC may be applied to various fields such as a non-polluting power source for an assembling type power generation unit, a portable power unit, and a military power unit. Therefore, the duel cells are extensively researched.
The power generation principle of the PEMFC is described hereunder. When hydrogen gas flows in the direction of cathode, hydrogen gas is decomposed into electrons and hydrogen ions (proton) in a catalyst. When the hydrogen ions are moved through the high molecular electrolyte membrane in the center of the fuel cell, the electrons, oxygen ions and hydrogen ions are combined at an anode for thereby producing water.
The electrons generated at the cathode do not move through the electrolyte membrane but move to the anode through an external circuit. Power and water are produced through the above procedures.
As shown in FIG. 4, as the elements belonging to the PEMFC, there is provided a flow path so that externally provided fuel gases (hydrogen, oxygen) are effectively flown to the electrode. There are further provided a separator 100 for moving generated electrons to an electrical circuit, a gas diffusion media 102 for uniformly diffusing fuel gases to the electrode film 101 and effectively discharging water produced based on an electrical chemical reaction, an electrode (not shown) for carrying a catalyst layer capable of generating an electrical chemical reaction of fuel gas, an electrolyte membrane (not shown) operating as a moving medium and preventing a fuel gas from being crossed over and preventing a short circuit, and a gasket 104 for externally protecting a fuel cell and a fuel gas in the interior of cell and other harmful materials from being discharged to the outside.
Reference numeral 103 represents an engaging bolt, and 105 represents an engaging end plate.
The PEMFC is an apparatus capable of generating power based on an electrical chemical reaction of fuel gas in the unit cell. In particular, entry into the fuel cell by any undesired foreign substance from an external environment is prevented based on a disconnection, e.g., a sealing barrier. In addition, tight sealing is provided to avoid any safety accidents by preventing hydrogen and fuel gases from being discharged to the outside.
The fuel cell gasket 104 is capable of maintaining a constant gap between the separator 100 and the electrode membrane 101, and also plays a role in uniformly distributing a fuel gas flowing into the separator. The reaction generation substance is easily removed. In particular, an electrical contact is maintained between the gas diffusion membrane 102 and the separator 100. The flow of electrons generated by the electrical chemical reaction is effectively enabled.
What is required in the industry is a gasket of the PEMFC wherein the physical property does not deteriorate under a harsh environment condition in the interior of the fuel cell and under an acidic environment condition of PH 1˜2. Additionally, as a low molecular weight problem in the material occurs, and as an additive and other ions are eluted, the electrical chemical reaction should be prevented from being interfered. The gasket should be usable in a wide range of temperature condition, namely, −40° C. through 120° C.
In addition, it is advantageous if the gasket is durable, and can uniformly maintain the gap as the time of use is passed for thereby preventing a decrease of performance of a fuel cell.
The gasket of a conventional PEMFC is generally classified into two kinds.
Namely, the PEMFC is classified into a solid gasket and a liquid gasket. In the case of the solid gasket, fluorine and silicon rubbers are compressed and formed and are adhered to the separator. In the case of the liquid gasket, the liquid gasket is coated on the separator or the electrode film and is hardened for thereby being adapted to the fuel cell. The fabrication process is relatively simple compared to the solid gasket. The fabrication cost is decreased, so that it can be widely used.
In the case of the conventional solid gasket, a rubber gasket having a certain thickness is fabricated and disengaged, and then is engaged to the separator of the fuel cell. For example, according to the Japanese patent laid-open No. Hei 9-507802, the solid gasket is fabricated using a combined material that a soft polytetrafluoroethylene layer is combined with a hard fluoropolymer. According to the Korean registered utility model No. 229074, a coupling protrusion is formed on a surface of a sealant for enhancing a coupling force with the sealant into which a porous carbon plate is inserted.
However, in the above method, it is impossible to fabricate an accurate conventional solid rubber gasket having a tolerance of below 0.011 mm using the metallic pattern. For example, when the rubber gasket having a thickness of below 0.5 mm is fabricated, it is impossible to obtain uniform sealing from a gasket commercially manufactured by this process.
In addition, the process for separating the finished gasket from the metallic pattern is difficult. Therefore, it is not easy to fabricate the soft gasket having a hardness shore of less than A 50 (HS). It is impossible to optimize the hardness of the gasket for a fuel cell structure.
Therefore, in the case of the solid gasket, it is so hard that it may cause transformation of a separator. Therefore, it becomes necessary to maintain engaging pressure at a relatively high level thus enlarging the engaging mechanism rather excessively during the stacking process of the fuel cell and also reducing the output density of the fuel cell.
In order to enhance the sealing property, various types of gaskets have been introduced and applied to the contact portions of the gaskets. In this case, the upper and lower gaskets are not accurately arranged, so that a certain torsion may occur. In the case that a few or a few hundreds of fuel cells are stacked, there may be a difference in sealing in each cell, so it is impossible to achieve a desired reliability.
According to the Japanese patent laid-open No. Hei 2000-12054, the liquid gasket is implemented using a liquid material in such a manner that a PTFE micro powder that is mixed with a liquid perfluoro rubber having a viscosity of below 105 poises.
However, in the case of the above liquid gasket, the FIPG (Formed-in-place-gasket) is generally adapted. In this case, the liquid gasket is directly coated to the separator. Therefore, the engaging process is simple. The process is simple, and the productivity is good. However, since the gas diffusion film, electrode film, etc. are stacked in a state that the liquid gasket is not hardened after it is directly coated on the separator, extreme cautions are required during the stacking process. It is difficult to maintain uniform intervals among cells. Since the hardening process is performed by the unit cell, the performance of the ion switch film may be decreased during a hardening process of high temperature.